1. Field of the Invention
The present invention relates to the control of electric motors and, more particularly, to a method and apparatus for detecting aberrant operating conditions of electric motors used in the drive systems of injection molding machines, and for deactivating such motors when the aberrant conditions are detected.
2. Description of the Related Art
Various electrical and mechanical devices are commonly used in conjunction with an electric motor so that the motor produces the desired effect for the particular application. However, these electrical and mechanical devices can sometimes malfunction causing the motor to produce an erroneous result. For example, one such error occurs when the motor is not properly connected to the machine element which is driven by the motor. The motor is connected to the driven elements by various mechanical connecting devices such as drive belts and pulleys; if any of these devices are loose or broken, or if the driven element is damaged, the machine will not function properly.
Another error which can occur in machines utilizing electric motors involves the various measurement devices used in the machines. For example, a linear position device, such as a linear displacement transducer, can be used to measure the position of the axis, and a resolver can be used to measure the angular position and/or velocity of the motor shaft. If either of these devices is loose or broken, the machine will not produce the desired response.
In addition, the software and electrical and mechanical control devices that control the machine may be faulty, thereby causing a malfunction. Furthermore, the operator of the machine may provide erroneous inputs to the machine, which, if undetected, can cause undesirable results.
Such erroneous conditions can be particularly problematic in an injection molding machine. In electrically driven injection molding machines, there may be as many as three or more electric motors used to drive machine elements for the various operating axes of the machine. For example, an electro-mechanically driven injection molding machine would likely have separate electric motors to power the clamp, the injection platen, and the ejection plate. If any of the above-noted faulty conditions occur, inconsistent molding machine performance, and even damage to the machine, can result. For example, a faulty electric motor in an injection molding machine might exceed its programmed velocity ("runaway") and cause the machine elements of the associated machine axis to interfere with or "crash" into another part in the machine, thereby causing potentially significant damage to machine components and, perhaps, the motor itself. Moreover, the molding machine would be unavailable for use while it is being repaired, thereby increasing the damage caused by the "runaway" motor.
It has been known to use limit switches in injection molding machines to prevent some malfunctions. Limit switches can detect when an axis is driven past a particular point beyond which it should not be permitted to go, and can shut off the motor accordingly. However, while limit switches can prevent some machine failures, they are not without disadvantages. First, they can only detect errors in axis position and not errors in motor velocity. Accordingly, even though the limit switch will shut off the motor if the axis is driven past the critical points, often times the motor will still be running at an unacceptably high speed, and the axis will proceed to "crash" even though the limit switch has been tripped. Second, limit switches cannot verify that the measured axis position is correct, but merely detect whether the axis has been driven beyond its predetermined maximum and minimum stroke positions. Thus, limit switches cannot really monitor machine operation on a continuing basis to detect errors in motor speed and/or machine axis position and, accordingly, often cannot prevent machine damage and malfunction.
Thus, there remains a need for an improved control system which continually monitors the machine for aberrant operating conditions and which can shut down the faulty drive and/or the machine as soon as the aberrant condition is detected. More specifically, there is a need to monitor the machine for errors in the position of machine elements, to verify that machine elements are at their expected location and operating within predetermined limits, such that motor drive coupling faults, erroneous sensing devices, and other malfunctions can be detected. Moreover, it is desired to monitor the machine continually for errors in motor velocity as defined by a velocity operating envelope, so that damage and poor performance can be prevented. It is also desirable to allow for changes in the velocity operating envelope with respect to axis position and/or velocity command signals, so that optimal motor control can be achieved.